The invention relates generally to furnaces, and is particularly directed to a system for controlling the energy released by a furnace so as to efficiently heat a structure to a desired temperature.
Conventional furnaces, such as those used to heat homes, are designed to heat a structure to approximately 70.degree. F. when the outside air is at its lowest expected temperature, zero degrees, for example. When the outside air is at its lowest expected temperature, the furnace runs continuously at its maximum output. When the outside air temperature is higher than its lowest expected temperature, the furnace runs full on for a while, shuts down for a while, and continuously alternates between full on and full off.
Because of the fact that a conventional furnace runs at full on when it is ignited, the gasses in the flue tend to become overheated. Consequently, flue losses which are proportional to the square of the difference between outside air temperature and flue temperature increase dramatically. Particularly on warmer days is the conventional furnace inefficient because of the fact that it is full on when ignited and much of the heat generated is sent up the flue.
Two additional problems with conventional furnaces arise because of the fact that such a furnace cycles between full on and full off. First, the air heating the structure is frequently warmer than it needs to be, thereby lowering the humidity in the interior of the structure below a level which is comfortable. Secondly, the cycling of the furnace between full on and full off tends to reduce the life of the furnace because of induced stresses from changes in the temperature of the furnace. In addition, moving parts tend to wear out prematurely. All these problems, in addition to the known problem of a period of overheating followed by a period of cooling, render conventional furnaces unsatisfactory. In the present climate of fuel shortages, furnace inefficiency is particularly troublesome.